1. Field of the Invention
The present invention relates to a communication apparatus (communication system), transmitter and receiver, and communication method, more particularly relates to a digital communication apparatus (system) for multi-carrier modulation, a wireless transmitter (wireless transmitting apparatus) and a wireless receiver (wireless receiving apparatus) used in a digital communication apparatus (system), and a communication method of the same.
More specifically, the present invention relates to a signal component demultiplexing apparatus for demultiplexing a multi-carrier signal multiplexed by orthogonal frequency division multiplexing (OFDM) to a symbol series, a filter apparatus for extracting specific symbols from a multi-carrier signal, and a signal receiving apparatus having these signal component demultiplexing apparatus, filter apparatus, etc.
2. Description of the Related Art
As an example of a signal modulated by OFDM, one of a digital audio broadcasting (DAB) system will be described.
The DAB system is known as a high quality digital audio terrestrial broadcasting method enabling mobile reception developed by the EUREKA 147 project. Progress is being made in commercialization of digital satellite audio broadcasting using the DAB system for the satellite broadcasting.
As the modulation method used in such a digital communication system (apparatus), OFDM has been proposed due to its tolerance to multi-path fading, ghosts, etc.
OFDM is a multi-carrier modulation method usually using tens to hundreds of orthogonal carriers. Each carrier is modulated by a modulation method such as quadrature amplitude modulation (QAM) or phase shift keying (PSK).
In the DAB system etc., digital audio signals of multiple channels are transmitted by multi-carrier communication.
FIGS. 21A and 21B are views of an example of the configuration of a digital wireless communication system used in a DAB system or the like using OFDM as the multi-carrier modulation method. FIGS. 21A and 21B illustrate parts of the DAB system in a simple form.
In the following explanation, the DAB system will be illustrated and the explanation made focusing on multiplexing.
A wireless transmitting apparatus 10 of an OFDM type wireless communication system illustrated in FIG. 21A has an encoder circuit 11, a symbol mapping circuit 12, a multiplexer (signal multiplexing circuit) 13, a frequency interleave circuit 14, an inverse fast Fourier transform (IFFT) circuit 15, a wireless transmitter circuit 16, and an antenna 17.
An information bit stream is encoded, interleaved, and otherwise processed in the encoder circuit 11, then its bits are mapped to transmission symbols in the symbol mapping circuit 12. This work is separately carried out for every channel. In the example shown in FIG. 21A, for example, 64 ksps (symbols/sec) of symbols are created per channel.
These symbol streams are simply connected in series in the multiplexer 13 to form a multiplexed symbol stream. For example, if 18 channels of 64 ksps per channel are multiplexed, the transmission rate of the multiplexed symbol stream becomes 1152 ksps (=18×64 ksps).
The symbols of the multiplexed symbol stream are rearranged by frequency interleaving in the frequency interleave circuit 14. The symbols of each channel are dispersed by this work.
Next, the dispersed symbols of the symbol stream are arranged on the frequency axis, then the symbol expressions on the frequency axis are transformed to symbols on the time axis by the IFFT processing in the inverse fast Fourier transform (IFFT) circuit 15, which are then sent from the transmitter circuit 16 via the antenna 17 into the air.
An example of the symbol string comprised of the six carriers formed into a multi-carrier signal output from the transmitting apparatus 10 is illustrated in FIG. 22.
Up until now, specific symbols among a plurality of symbols (symbol series) formed into the above multi-carrier signal have not been solely extracted.
Therefore, we suppose the wireless signal receiver extracts the intended symbols or carrier components from the symbol series illustrated in FIG. 22 using an existing technique.
FIG. 23 is a view of a first method for demultiplexing a multi-carrier signal.
In this method, a plurality of band pass filters having frequency band characteristics of the corresponding carriers are provided. The corresponding symbols are extracted by these band pass filters. As such filters, use can be made of for example comb type filters.
However, such a method is unsuitable for demultiplexing symbols of a modulation method such as OFDM where the carriers are crammed together. Namely, with a modulation method using OFDM, a large number of carriers are crammed in a certain frequency band, therefore adjoining signal components cannot be sufficiently isolated. Accordingly, each band pass filter must have a sharp frequency characteristic in order to discriminate between carrier signals of adjoining frequencies.
For example, it is difficult to prepare various types of high precision filters such as comb type filters which have such sharp frequency characteristics. Further, this becomes considerably expensive in terms of price. Therefore, it is difficult to realize this.
FIG. 24 is a view of a second method of demultiplexing a multi-carrier signal.
In FIG. 24, a Fast Fourier transform (FFT) is applied to a signal received at a receiver circuit 22 in a Fast Fourier transform (FFT) circuit 23 to create a received symbol series arranged on the frequency axis. The symbol series is demultiplexed to separate symbols in a demultiplexer (signal demultiplexing apparatus) 29. Due to this, it is possible to select only specific symbols.
In this method, however, even when extracting specific symbols, the fast Fourier transform is applied to all symbols. Therefore, a complex FFT circuit 23 must be provided, so the hardware configuration becomes complex.
FIG. 25 is a schematic view of the configuration when extracting only carrier signal components at constant intervals. In FIG. 25, a plurality of band pass filters having a plurality of different band pass characteristics are provided. Signals limited in bands by the filters are added to each other at adder circuits 28A and 28B to obtain the intended signal. In this case as well, as the band pass filters, for example, comb type filters can be used.
However, in the same way as with the method of FIG. 23, since it is a multi-carrier method, this method also suffers from the disadvantages that carriers are crammed together, so the signal components cannot be sufficiently isolated. Also, it becomes difficult to prepare high precision filters having sharp frequency characteristics from the cost perspective etc.
FIG. 21B is a schematic view of the configuration of a wireless signal receiver in the DAB system illustrated in FIG. 21A.
A wireless receiving apparatus 20 of an ODFM wireless communication system 1 of FIG. 21B has an antenna 21, a receiver circuit 22, a Fast Fourier transform (FFT) circuit 23, a symbol selection circuit 24, a bit extraction circuit 25, and a decoding circuit 26.
By transforming the frequency of the signal of the intended frequency band received at the antenna 21 in the receiver circuit 22 and extracting only the baseband signal component, a baseband signal is obtained. The thus obtained baseband signal is expressed on the time axis of the signal with the information arranged on the frequency axis. Therefore, FFT processing is carried out in the FFT circuit 23 to extract subcarriers arranged on the frequency axis.
At this time, the symbols output by the FFT processing consist of the group of subcarriers of the signal bands received as a whole (for example, in the present example, containing 1152 ksps worth of information).
The symbol selection circuit 24 extracts the symbols from the group of subcarriers from the positions of the symbols of the intended channel arranged by the frequency interleaving at the transmission side illustrated in FIG. 21A. By this, the 64 kbps of information of the intended channel is extracted.
The received bit stream is extracted from among the symbol stream of the intended channel obtained in this way in the bit extraction circuit 25 to obtain the encoded bit stream, then this is decoded at the decoding circuit 26 to obtain the information bit stream of the intended channel.
Summarizing the disadvantages to be solved by the invention, in this way, in OFDM, multiplexing is carried out by allocating symbols of different channels to different subcarriers, but this means that the wireless receiving apparatus 20 receives a multiplexed signal of all channels transmitted and, further, that the FFT circuit 23 extracts the symbols of all of the channels, then the symbol selection circuit 24 selects the channel. Therefore, the FFT circuit 23 performs FFT processing entailing computations far exceeding the amount required for the originally required one channel's worth of information.
Namely, this means that the FFT circuit 23 performs the FFT signal processing for even channels which the wireless receiving apparatus 20 does not desire, so there is a disadvantage in that the FFT circuit 23 becomes unnecessarily large in scale.
As a method of solving this disadvantage, the present inventors have proposed the invention disclosed in for example Japanese laid open patent No. 2000-332722 published on Nov. 30, 2000. In the invention disclosed in Japanese laid open patent No. 2000-332722, circuits for demultiplexing a symbol string for every alternate subcarrier from the symbol series are provided in multiple stages hierarchically by a branching method.